This application is a competitive renewal of a project directed toward furthering the understanding of the basic scattering mechanisms of ultrasound in biological tissues, particularly in blood. This knowledge is needed for the better interpretation of ultrasonic images and the further development of the modality. Data on ultrasonic scattering properties of blood have an additional significance in that they are essential for the design of ultrasonic Doppler devices as well. Recently there has been a growing interesting this subject because of the introduction and emerging clinical applications of intravascular ultrasonic imaging devices which typically are operated at frequencies higher than 20 MHz and an attempt by a scanner manufacturer to produce images, that map the Doppler signal strength, or "ultrasound angiograms". In the former application, in order to be able to visualize the blood vessel wall, ultrasound pulses emitted by ultrasonic elements mounted on the tip of a catheter have to traverse blood to reach the vessel lumen. As a result, an understanding of high frequency blood scattering properties is of importance in design considerations. Moreover, these devices may offer unique and exciting opportunities for studying blood flow in vivo and perhaps for continuous assaying of certain blood properties in silL' if the ultrasonic scattering properties of blood at these frequencies can be appropriately utilized. In the latter application, Doppler power instead of Doppler shift from blood is used-to produce a color map to indicate tissue perfusion or blood supply. The advantages of this approach over conventional color Doppler are that it is angle independent and it is less affected by noise. The future of this technique is critically upon a clear understanding of relationship between Doppler power from blood and hematological and hemodynamic parameters. There are three specific aims for the next grant period. (1) We will continue the investigation of ultrasonic scattering properties of whole blood and erythrocyte suspensions under well-defined flow conditions with more emphasis on pulsatile flow both in vitro in a mock flow loop and in vivo on dogs. We will use a modified intravascular imaging device for measuring ultrasonic scattering from blood in situ inside the flow conduit and inside the blood vessels. We will extend the in vitro scattering measurements to 60 MHz. (2) We will explore the feasibility of using this knowledge as a basis for developing new methods for assaying such blood properties as fibrinogen concentration and erythrocyte aggregation. (3) We will better define the underlying mechanisms responsible for ultrasonic scattering in biological tissues, in particular, skeletal muscle.